Due to recent advances in genetic and cell engineering technologies, proteins known to exhibit various pharmacological actions in vivo are capable of production in large amounts for pharmaceutical applications. Such proteins include erythropoietin (EPO), granulocyte colony-stimulating factor (G-CSF), interferons (alpha, beta, gamma, consensus), tumor necrosis factor binding protein (TNFbp), interleukin-1 receptor antagonist (IL-1ra), brain-derived neurotrophic factor (BDNF), kerantinocyte growth factor (KGF), stem cell factor (SCF), megakaryocyte growth differentiation factor (MGDF), osteoprotegerin (OPG), glial cell line derived neurotrophic factor (GDNF) and obesity protein (OB protein). OB protein may also be referred to herein as leptin.
Granulocyte colony stimulating factor (G-CSF) is a glycoprotein which induces differentiation of hemapoietic precursor cells to neutrophils, and stimulates the activity of mature neutrophils. human G-CSF (rhG-CSF), expressed in E. coli, contains 175 amino acids, has a molecular weight of 18,798 Da, and is biologically active. Currently, Filgrastim, a recombinant G-CSF, is available for therapeutic use.
The structure of G-CSF under various conditions has been extensively studied; Lu et al., J. Biol. Chem. Vol. 267, 8770-8777 (1992), and the three-dimensional structure of rhG-CSF has recently been determined by x-ray crystallography. G-CSF is a member of a class of growth factors sharing a common structural motif of a four .alpha.-helix bundle with two long crossover connections; Hill et al., P.N.A.S. USA, Vol. 90, 5167-5171 (1993). This family includes GM-CSF, growth hormone, interleukin-2, interleukin-4, and interferon .beta.. The extent of secondary structure is sensitive to the solvent pH, where the protein acquires an even higher degree of alpha helical content at acidic pH; Lu et al., Arch. Biochem. Biophys., 286, 81-92 (1989).
Leptin is active in vivo in both ob/ob mutant mice (mice obese due to a defect in the production of the OB gene product) as well as in normal, wild type mice. The biological activity manifests itself in, among other things, weight loss. See generally, Barinaga, "Obese" Protein Slims Mice, Science 269: 475-476 (1995) and Friedman, "The Alphabet of Weight Control," Nature 385: 119-120 (1997). It is known, for instance, that in ob/ob mutant mice, administration of leptin results in a decrease in serum insulin levels, and serum glucose levels. It is also known that administration of leptin results in a decrease in body fat. This was observed in both ob/ob mutant mice, as well as non-obese normal mice. Pelleymounter et al., Science 269: 540-543 (1995); Halaas et al., Science 269: 543-546 (1995). See also, Campfield et al., Science 269: 546-549 (1995) (Peripheral and central administration of microgram doses of leptin reduced food intake and body weight of ob/ob and diet-induced obese mice but not in db/db obese mice.) In none of these reports have toxicities been observed, even at the highest doses.
Preliminary leptin induced weight loss experiments in animal models predict the need for a high concentration leptin formulation with chronic administration to effectively treat human obesity. Dosages in the milligram protein per kilogram body weight range, such as 0.5 or 1.0 mg/kg/day or below, are desirable for injection of therapeutically effective amounts into larger mammals, such as humans. An increase in protein concentration is thus necessary to avoid injection of large volumes, which can be uncomfortable or possibly painful to the patient.
Unfortunately, for preparation of a pharmaceutical composition for injection in humans, it has been observed that the leptin amino acid sequence is insoluble at physiologic pH at relatively high concentrations, such as above about 2 mg active protein/milliliter of liquid. The poor solubility of leptin under physiological conditions appears to contribute to the formation of leptin precipitates at the injection site in a concentration dependent manner when high dosages are administered in a low pH formulation. Associated with the observed leptin precipitates is an inflammatory response at the injection site which includes a mixed cell infiltrate characterized by the presence of eosinophils, macrophages and giant cells.
To date, there have been no reports of stable preparations of human OB protein at concentrations of at least about 2 mg/ml at physiologic pH, and further, no reports of stable concentrations of active human OB protein at least about 50 mg/ml or above. The development of leptin forms which would allow for high dosage without the aforementioned problems would be of great benefit. It is therefore one object of the present invention to provide improved forms of leptin by way of site-specific chemical modification of the protein.
There are several methods of chemical modification of useful therapeutic proteins which have been reported. One such method, succinylation, involves the conjugation of one or more succinyl moieties to a biologically active protein. Classic approaches to succinylation traditionally employ alkaline reaction conditions with very large excesses of succinic anhydride. The resultant succinyl-protein conjugates are typically modified at multiple sites, often show altered tertiary and quaternary structures, and occasionally are inactivated. The properties of various succinylated proteins are described in Holcenberg et al., J. Biol. Chem, 250:4165-4170 (1975), and WO 88/01511 (and references cited therein), published Mar. 10, 1988. Importantly, none of the cited references describe methods wherein the biologically active protein is monosuccinylated exclusively at the N-terminus of the protein, and wherein the resultant composition exhibits improved solubility and improved injection site toxicity's.
Diethylenetriaminepentaacetic acid anhydride (DTPA) and ethylenediaminetetraacetic acid dianhydride (hereinafter referred to as EDTA.sup.2) have classically been used to introduce metal chelation sites into proteins for the purpose of radiolabeling. Similar to succinylation, modification with DTPA and/or EDTA.sup.2 typically occurs at multiple sites throughout the molecule and changes the charge and isoelectric point of the modified protein. To date, there have been no reports of DTPA- and/or EDTA.sup.2 -protein monomers and dimers which exhibit improved solubility and improved injection site toxicity's.